SO i did and experiment on a irregular shape object .and if you want to find the density of a marble you have to weigh it first on a tripple beam balance then you record what the mass was. then u use...
mass divided by volume will give the density...
Divide the mass by the volume.
Drop the object into a water filled graduated cylinder
Well you Find the density by doing Mass/Volume. If The objects density is less than 1 it will float